OSHA's excavation standard, 29 CFR 1926 Subpart P, is short but full of specific numbers. Learn these and you know most of the rules that inspectors look for and crews depend on.
The numbers at a glance
| Number | Rule | Where |
|---|---|---|
| 2 feet | Keep spoil, materials and equipment at least 2 feet from the edge (or use retaining devices) | 1926.651(j)(2) |
| 2 feet | You may dig up to 2 feet below a shield or support system only if it's rated for the full depth and no soil is lost | 1926.652(e)(2), (g)(2) |
| 3 feet | Ladder rails extend at least 3 feet above the top of the trench | 1926.1053(b)(1) |
| 4 feet | Trenches 4 feet or deeper need a ladder, stair or ramp | 1926.651(c)(2) |
| 4 feet | Test the air before entering excavations deeper than 4 feet where a hazardous atmosphere could exist | 1926.651(g)(1)(i) |
| 5 feet | Excavations 5 feet or deeper need a protective system unless entirely in stable rock | 1926.652(a)(1) |
| 6 feet | Walkways 6 feet or more above lower levels need guardrails; hidden excavations 6 feet or deeper need barricades | 1926.651(l), 1926.501(b)(7) |
| 18 inches | A shield below a sloped section must extend at least 18 inches above the vertical side | Appendix B |
| 19.5% | Minimum oxygen level | 1926.651(g)(1) |
| 20% | Keep flammable gas below 20% of its lower flammable limit | 1926.651(g)(1)(iii) |
| 15 feet | A trench is no more than 15 feet wide at the bottom | 1926.650(b) |
| 20 feet | Protective systems for excavations deeper than 20 feet must be designed by a registered professional engineer | 1926.652(b), (c) |
| 25 feet | Maximum lateral travel to the nearest means of egress in trenches 4 feet or deeper | 1926.651(c)(2) |
| 24 hours | Utilities that don't respond within 24 hours: proceed with caution using detection equipment | 1926.651(b)(2) |
The slopes
| Soil | Max slope (H:V) | Angle |
|---|---|---|
| Stable rock | Vertical | 90° |
| Type A | ¾:1 | 53° |
| Type B | 1:1 | 45° |
| Type C | 1½:1 | 34° |
Short-term Type A excavations (12 feet or less, open 24 hours or less) may use ½:1. Calculate your setbacks with the trench slope calculator.
The things that don't have a number
Some of the most important rules are about people and timing, not measurements:
- A competent person must inspect before work, as needed during the shift, and after every rainstorm or hazard-increasing event (1926.651(k)).
- Workers must be removed when there are signs of a cave-in, protective system failure or hazardous atmosphere.
- No one may be under loads handled by lifting or digging equipment (1926.651(e)).
- Mobile equipment near an edge the operator can't see needs a warning system (1926.651(f)).
- Workers exposed to traffic wear high-visibility garments (1926.651(d)).
- Utilities must be located before digging and exposed by safe means (1926.651(b)).
- Water accumulation requires precautions and competent person monitoring of pumps (1926.651(h)).
Test yourself
Take our free trench safety practice test or start with lesson 1.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1926 Subpart P. It does not replace the site-specific training and competent person your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.